1. Field of the Invention
This invention relates to an ion implantation method that can improve a uniformity of doping to trench sidewalls of a semiconductor substrate having a trench structure, and a method of manufacturing a semiconductor device making use of the implantation method.
2. Description of the Prior Art
In conventional ion implantation to a trench structure, trench sidewalls that face in various directions are uniformly doped with impurities by a method in which an ion beam scans the surface of a wafer and the wafer is continuously rotated in the state that the beam is inclined with respect to the direction of a perpendicular line of the wafer, or a method in which a wafer is stepwise rotated in intermittently rotating step ion implantation. In these methods, however, a problem may arise, which is a problem of an error in angles. On the latter method, the present inventor and his colleagues have made a report in IEEE Transaction on Electron Devices, Vol. ED-34, No. 2, p. 356, 1987.
The error in angles in the conventional ion implantation to a trench structure will be explained below with reference to FIG. 7. In an ion implantation apparatus, scanning is electrostatically carried out through a scanning electrode 7 provided at a certain point on the beam line as shown in FIG. 7, and hence a delicate distribution of implantation angles is produced on the angles in a wafer 1. This results in a loss of uniformity of doping to the trench sidewalls. In ordinary ion implantation apparatus presently available, the error is produced at an angle of about .+-.3.degree.. This angle, when, for example, ions are implanted at an angle of 7.degree., brings about a difference of from 4.degree. to 10.degree. in implantation angles from one end to the other end of the wafer. Hence, the amount of implantation may greatly differ at sidewalls, depending on the scanning positions on the wafer.
In conventional ion implantation methods, when the implantation angle is set at 7.degree. at the central part and a sidewall 3 and a sidewall 5 of each trench 2 in a substrate 1 are set at 4.degree. and 10.degree., respectively, with respect to the surface of the substrate, the amount of implantation to sidewalls, dependent upon angles, can be calculated to find that ions are implanted in a larger amount of simply as many as about 1.42 times to the sidewall having an angle of 10.degree. with respect to the sidewall of a trench at the central part. At the sidewall having an angle of 4.degree., it follows that ions are implanted in an amount of 0.57 time.
On the other hand, an apparatus in which the scanning velocity of an ion beam can be varied depending on the position at which the beam scans the surface has been made commercially available for the purpose of enhancing the uniformity of doping. This is reported in M. Sasaki et al, Nuclear Instrument and Method in Physics Research B37/38 (1989) p. 469. This method, however, is a method in which a non-uniform beam density over the whole wafer because of the spread of a beam is corrected. The method can be effective for making uniform the doping to the whole surface of the wafer, but may more worsen the uniformity of doping to the sidewalls of a trench, which are highly sensitively affected by angles.
It is also attempted to provide a method in which a wafer is irradiated with a perfectly parallel beam. This method, however, requires a complicated structure for its apparatus, resulting in a very high price of the apparatus. Process cost increases accordingly. In addition, even if a parallel beam can be produced, there is a limit to its accuracy, and it is very difficult to give an angle of .+-.1.degree. or less.